From 0a86850ba37470837cbd19b33679763851c13b73 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Wed, 5 Aug 2020 09:35:17 +0100
Subject: [PATCH] Stop the parent process flushing the logs on exit (#8012)

This solves the problem that the first few lines are logged twice on matrix.org. Hopefully the comments explain it.
---
 changelog.d/8011.bugfix   |  1 +
 changelog.d/8011.misc     |  1 -
 changelog.d/8012.bugfix   |  1 +
 synapse/util/daemonize.py | 10 ++++++++--
 4 files changed, 10 insertions(+), 3 deletions(-)
 create mode 100644 changelog.d/8011.bugfix
 delete mode 100644 changelog.d/8011.misc
 create mode 100644 changelog.d/8012.bugfix

diff --git a/changelog.d/8011.bugfix b/changelog.d/8011.bugfix
new file mode 100644
index 000000000000..c673040de938
--- /dev/null
+++ b/changelog.d/8011.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug which caused two copies of some log lines to be written when synctl was used along with a MemoryHandler logger.
diff --git a/changelog.d/8011.misc b/changelog.d/8011.misc
deleted file mode 100644
index dfeb4bdaf1c6..000000000000
--- a/changelog.d/8011.misc
+++ /dev/null
@@ -1 +0,0 @@
-Replace daemonize library with a local implementation.
diff --git a/changelog.d/8012.bugfix b/changelog.d/8012.bugfix
new file mode 100644
index 000000000000..c673040de938
--- /dev/null
+++ b/changelog.d/8012.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug which caused two copies of some log lines to be written when synctl was used along with a MemoryHandler logger.
diff --git a/synapse/util/daemonize.py b/synapse/util/daemonize.py
index a7913fa1afa9..23393cf49bb7 100644
--- a/synapse/util/daemonize.py
+++ b/synapse/util/daemonize.py
@@ -60,8 +60,14 @@ def daemonize_process(pid_file: str, logger: logging.Logger, chdir: str = "/") -
     process_id = os.fork()
 
     if process_id != 0:
-        # parent process
-        sys.exit(0)
+        # parent process: exit.
+
+        # we use os._exit to avoid running the atexit handlers. In particular, that
+        # means we don't flush the logs. This is important because if we are using
+        # a MemoryHandler, we could have logs buffered which are now buffered in both
+        # the main and the child process, so if we let the main process flush the logs,
+        # we'll get two copies.
+        os._exit(0)
 
     # This is the child process. Continue.